If removing PP works, then my guess is that something in that module is what's causing problems.
I wouldn't suspect any hooks to be flawed then.
My suggestion would be to make sure your PP-files are un-altered and that your configurations in both the module and at PP is as they should be. Even though the return-URL is correct by default, there could be something else that was wrong.
If I understood the errors correct (which I very well may not have), it seemed to expect a class with no name, which it of course should not. To my understanding, this means that something is calling that class.
You said your host did something? Maybe it's worth finding out what.
//Daniel